1. Field of the Invention
The present invention relates to an electrical conductive member, a charging member using the electrical conductive member, a process cartridge using the charging member, and an image forming apparatus using the process cartridge.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copying machine, a laser beam printer, or a facsimile has been conventionally provided with a charging member that performs an electrification process on a photoreceptor drum (an image carrier), and a transfer member that performs a transfer process on a toner on the photoreceptor drum. An electrical conductive member is used as the charging member or the transfer member.
FIG. 1 illustrates a schematic structure of an image forming apparatus. The image forming apparatus 1 is composed of a photoreceptor drum 4 serving as an image carrier where an electrostatic latent image is formed; a charger roller 2 serving as a charging member that performs an electrification process on the photoreceptor drum 4; a developing roller 6 that causes a toner 5 to adhere to the electrostatic latent image on the photoreceptor drum 4; a transfer roller 7 that transfers a toner image on the photoreceptor drum 4 to a recording paper S; and a cleaning blade 8 for cleaning the photoreceptor drum 4 after the transfer process. Reference numeral 9 denotes waste toner discharged by a cleaning member for removing residual toner on the photoreceptor; 10, a developing unit; and 11, a cleaning unit. Incidentally, functional units generally required for other electrophotographic processes are omitted from FIG. 1 as their explanation is unnecessary.
The charger roller 2 is powered by a power pack (not shown) to electrify the photoreceptor drum 4 to a desired potential. This photoreceptor drum 4 is rotated in the direction of arrow A by a drive unit (not shown). A surface electrometer (not shown) is installed just behind the charger roller 2 along the direction of the rotation of the photoreceptor drum 4 to measure a potential of a surface 4a of the photoreceptor drum 4.
The developing roller 6 attaches the toner to the electrified photoreceptor drum 4. The transfer roller 7 transfers the toner image on the photoreceptor 4 to the recording paper S. The cleaning blade 8 removes the residual toner on the photoreceptor drum 4 to clean the photoreceptor drum 4.
An image forming process of the image forming apparatus 1 is as follows. First, the surface 4a of the photoreceptor drum 4 is electrified to a negative high potential by the charger roller 2. Then, the surface 4a is exposed. Corresponding to an amount of light received by this exposure L, a potential distribution is formed on the surface 4a, and, as a result, an electrostatic latent image is formed on the surface 4a. 
When the photoreceptor drum 4 is rotated and a part where an electrostatic latent image on the surface 4a was formed passes the developing roller 6, the toner corresponding to the potential distribution adheres to the surface 4a. As a result, an electrostatic latent image is made visible as the toner image. This toner image is transcribed on a fed recording paper S by the transfer roller 7 with a prescribed timing, and the recording paper S is conveyed in the direction of arrow B toward a fixing unit (not shown).
Meanwhile, after the transcribing, the cleaning blade 8 is used to remove the residual toner from the photoreceptor drum 4 and clean the photoreceptor drum 4, accompanied by a quenching lamp (not shown) for removing the charge before shifting to the following image forming process.
A conventional electrification method employed in the image forming apparatus 1 is commonly known as a contact-type electrification method where the charger roller 2 is in contact with the photoreceptor drum 4. (for reference, see Japanese Application Publication Numbers S63-149668, Hei 1-211779 and Hei1-267667)
However, the conventional contact-type electrification method using the charger roller 2 has the following problems (1) to (5).
(1) The charger roller leaves traces thereof on the surface of the photoreceptor drum because a constituent substance of the charger roller exudes from the charger roller and adheres onto the surface of the photoreceptor drum.
(2) Static crackling is emitted because the charger roller 2 in contact with the photoreceptor drum vibrates when subjected to an alternating voltage.
(3) The charger roller undergoes a deterioration in electrification performance because the toner on the photoreceptor drum adheres to the charger roller (or, in particular, the above-mentioned exudation increases the likelihood of adhesion of the toner).
(4) The constituent substance of the charger roller is prone to adhere to the photoreceptor drum.
(5) The charger roller undergoes permanent deformation when the photoreceptor drum is not in use for a long time.
As a method for solving the above problems, instead of bringing the charger roller 2 into contact with the photoreceptor drum 4, there is proposed a proximity electrification method which involves bringing the charger roller 2 into proximity with the photoreceptor drum 4 (as disclosed in Japanese Application Publication Number Hei3-240076). The method is used to electrify the photoreceptor drum by applying a voltage to the charger roller 2 disposed facing the photoreceptor drum with a distance (a gap will be used hereinafter) of 50 μm to 300 μm in between when the charger roller and the photoreceptor drum are in closest proximity to each other.
Because no contact is provided between the charger roller 2 and the photoreceptor drum 4, the proximity electrification method does not present the problems inherent in the conventional contact-type electrification method, namely, “the adhesion of the constituent substance of the charger roller to the photoreceptor drum” and “the permanent deformation of the charger roller caused by the photoreceptor drum being out of use for a long time.” As for “the deterioration in the electrification performance of the charger roller due to the adhesion of toners,” the proximity electrification method is superior because a lesser amount of the toner is adhered to the charger roller.
The required characteristic properties of the charger roller for the proximity electrification method are different from those of the charger roller for the contact-type electrification method. The periphery of a core of the charger roller generally used for the contact-type electrification method is covered by an elastic member made of a vulcanized rubber, etc.
The charger roller is required to come in contact with the photoreceptor uniformly so as to uniformly electrify the photoreceptor when the contact-type electrification method is used.
In contrast, when the proximity electrification method is used instead of the contact-type electrification method, if the charger roller is made of the elastic member, problems may arise as follows.
(1) So as to generate the gap between the photoreceptor and the charger roller, it is required that a gap retaining member be disposed in close proximity to the photoreceptor, such as a spacer, in a non-image area at both ends of the charger roller. However, when the charger roller is made of the elastic member, due to a deformation of the elastic member, it is difficult to maintain a uniform gap width. Consequently, a fluctuation may be generated in an electrification potential or an irregular image may be caused by the fluctuation in the electrification potential.
(2) The elastic member made of the vulcanized rubber is prone to deform and decrease in quantity thereof with the lapse of time, which causes fluctuation in the width of the gap with the lapse of time.
In order to solve the above problems, it is proposed that a thermoplastic resin which is inelastic be used, which can maintain the gap width as constant. It is known that an electrification mechanism which charges the surface of the photoreceptor drum by the charger roller follows Paschen's law within a small space between the charger roller and the photoreceptor drum. It is necessary to control an electric resistance value of the thermoplastic resin within a semi-conductive range (about 106 Ωcm˜109 Ωcm) to make the photoreceptor drum function to maintain a prescribed electrification potential.
A known method for controlling the electric resistance value of the thermoplastic resin involves dispersing conductive pigments such as carbon black, etc. therein. However, when attempting to control the electric resistance value of the electric resistance adjusting layer within the semi-conductive range, because the electric resistance value fluctuates greatly, there occurs either a partial electrification deterioration or a local discharge (a leakage discharge) caused by an electron conduction. Consequently, a defective image is generated.
Another method to control the electric resistance value of the electric resistance adjusting layer is to add an ionic conductive material.
Since the ionic conductive material may be dispersed at the molecular level in a matrix resin, a fluctuation in the electric resistance value thereof is less than that when the conductive pigments are dispersed. Therefore, a resultant partial faulty electrification will not impair the image quality. However, the ionic conductive material having a low molecular weight such as an electrolyte salt is prone to bleed out to a surface of the matrix resin. When the electrolyte salt bleeds out to the surface of the charger roller, the adhesion of the toner will occur, which causes a defective image.
For the prevention of this bleed-out phenomenon, a polymeric ionic conductive material is proposed. Since the polymeric ionic conductive material is dispersed and immobilized at the molecular level in a matrix resin, the occurrence of the bleed-out to the surface of the matrix resin becomes infrequent. Although a polyamide-based elastomer, etc. is used as the polymeric ionic conductive material, since the electric resistance value of the electric resistance adjusting layer, made from such a material, is high, which makes it impossible to control the electric resistance value within the semi-conductive range only by using a polymeric ionic conductive material, the electrolyte salt is added so as to impart electrical conductivity.
Examples of the above-mentioned electrolyte salt include perchlorate such as a sodium perchlorate, a lithium perchlorate, etc. However, when the sodium perchlorate is used, because of a reaction thereof with moisture in the air on an ionic dissociation, a strongly-alkaline sodium hydroxide is generated. Consequently, the matrix resin is deteriorated with the lapse of time and (solvent) cracks are generated.
In order to prevent the cracks, the use of other electrolyte salts which do not generate a strongly-alkaline substance can be considered. To be specific, it is known that organic phosphonium salt can be added without the generation of the strongly-alkaline substance, which does not pose a problem in terms of the deterioration of the resin with the lapse of time and the generation of the cracks.
Concerning the conductive member, relevant technologies have been disclosed in Japanese Application Publication Number 2006-85006, Japanese Application Publication Number 2002-311687, Japanese Application Publication Number 2005-275412, Japanese Application Publication Number 2005-121982, Japanese Application Publication Number 2002-132019 and Japanese Application Publication Number 2005-91818.
As disclosed in Japanese Application Publication Number 2006-85006, a conductive rubber roller has a rubber layer formed around a conductive core material. The rubber layer is made of a foam containing 75 to 99.5 pts.wt. of acrylonitrile-butadiene rubber which contains 15 to 43 pts.wt. of nitrile, a 25 to 0.5 pts.wt. of polyethylene oxide-polypropylene oxide-acrylglycidyl ether ternary copolymer and 0.1 to 4.0 pts.wt. either of halogen-containing quaternary ammonium salts or of quaternary phosphonium salts.
As disclosed in Japanese Application Publication Number 2002-311687, an electrification roller is formed by applying an ionic conductive tube on an elastic layer. The ionic conductive tube is made from a polymer composition containing (A) a polymer selected from the group consisting of polyurethane, polyamide and polyester thermoplastic elastomers and (B) a quaternary phosphonium salt having at least three phenyl groups coupled to phosphorus atoms in a molecule thereof.
A conductive member, as disclosed in Japanese Application Publication Number 2005-275412, is obtained by molding a composition comprising a non-ether-based polyurethane, carbon black and bis (trifluoromethane sulfonyl) imide lithium.
As disclosed in Japanese Application Publication Number 2005-121982, a polyurethane foam is made by foam-curing the polyurethane foaming composition, which is a mixture of organic polyisocyanate, polyol, catalyst, foaming agent, and conductive material with inert gas, after mechanically stirring them. The conductive material contains an ionic conductive agent containing either potassium bis (trifluoromethane sulfonyl) imide or lithium bis (trifluoromethane sulfonyl)imido, or both. The Asker stiffness of the conductive roll is C5°-C80°, the resistance is 1×104˜1×108 Ω, and the mass density of the polyurethane foam is 0.1˜0.8 g/cm3.
In Japanese Application Publication Number 2002-132019, there is disclosed a charging member for a proximity electrification method. The charging member has an electric resistance adjusting layer consisting of a thermoplastic resin composition wherein polyetheresteramide components are dispersed as a polymeric ionic conductive material.
As disclosed in Japanese Application Publication Number 2005-91818, in a conductive rubber roller, a rubber layer is formed on a conductive core material. The conductive member is provided with a conductive supporter, an electric resistance adjusting layer installed on the peripheral surface thereof and a pair of gap retaining members firmly fixed to end surfaces of the electric resistance adjusting layer. A height difference is given between outer peripheral surfaces of the gap retaining members and the outer peripheral surface of the electric resistance adjusting layer so that a gap with a fixed width may be formed between the outer peripheral surface of the photoreceptor and that of the electric resistance adjusting layer when the conductive supporter is brought into contact with the photoreceptor, and outer peripheral surfaces of end parts of gap retaining members adjacent to the electric resistance adjusting layer are machined so as not to abut on the outside surface of the photoreceptor.
However, when the organic phosphonium salt is added alone, since a voltage dependency of the resistance value of the electric resistance adjusting layer is small, the resistance value rises when a high voltage is applied to a commercially-produced apparatus. Therefore, it is known that there are generated an irregular discharge and a defective image under the circumstances of low temperature and low humidity.
Therefore, via the addition of a fluorine-containing organic anion salt to the organic phosphonium salt, an absolute value of the resistance of the electric resistance adjusting layer is further lowered. It is known that even if the voltage dependency of the resistance value of the electric resistance adjusting layer is small, the irregular discharge would not occur under the circumstances of low temperature and low humidity.
Meanwhile, when a perchlorate is used to form the electric resistance adjusting layer, since a voltage dependency of a resistance value of the electric resistance adjusting layer is large, the resistance value rises remarkably due to a polarization of an electrolytic salt resulting from an energization over a long term. Therefore, it is known that an irregular discharge may occur after long-term use of the electric resistance adjusting layer.
In particular, it is known that when the charger roller is used by applying thereto a high voltage continuously, the resistance value of the electric resistance adjusting layer varies dramatically after long-term use compared with a resistance value thereof at an early stage.
In contrast, when the organic phosphonium salt and the fluorine-containing organic anion salt are used to form the electric resistance adjusting layer, since the voltage dependency of the resistance value of the electric resistance adjusting layer is small, the resistance value rises insignificantly due to the energization. Therefore, it is known that the irregular discharge does not occur even after long-term use of the electric resistance adjusting layer.
In view of the above problems, an object of the present invention is to provide an excellent electrical conductive member which is capable of maintaining a stable resistance value even after long-term use thereof without the generation of a defective image due to the irregular discharge, a process cartridge using the electrical conductive member, and an image forming apparatus using the process cartridge.